1. Field of the Invention
The present invention relates generally to the field of measuring and testing, and more specifically, to a test apparatus and method for determining the ignition performance of ignition devices, i.e., devices that produce flame.
2. Description of the Related Art
Ignition of propellant materials used in rocket motors, or munitions, or even automobile air bags, all start with an igniter or initiator which can be fired either electrically or mechanically. Percussion primers are one type of munitions application. Percussion primers are generally about 0.2 inch or less in diameter and consist of thin wall brass cups containing friction-sensitive pyrotechnic compositions and an internal anvil. Impacting and deflecting the bottom of one of the cups compresses the pyrotechnic composition against the anvil to achieve ignition, thereby outputting a quantity of heat, light, gas and burning particles.
Percussion primers have been in use for over one hundred years. Over this time, test methods and apparatuses have been devised to measure different aspects of an ignition output, such as heat, light, pressure, and burning particles. Heat output has been measured with calorimeters and light output has been measured with photocells and photographs of an ignition output's light and burning particles in a darkened room and with the aid of vented test volumes. Pressure measurements have been made by simply having an operator listen to relative sound of the output in vented test fixtures, and by measuring the pressure in closed test volumes.
Tests of the effect of burning particles are made in a vented volume and by a "standoff" method. In the latter, a primer is fired into a large, deep volume with a test propellant at the base. The depth is varied until ignition fails to occur.
One of the disadvantages common to the above-mentioned test methods and apparatuses is that virtually all system-level tests have been performed on a go/no-go basis. That is, a system or sub-system is assembled and then initiated. The results will show that the tested initiator, such as a percussion primer, either did or did not work properly. Thus, the tests do not provide sufficient information on actual ignition interfaces used in ignition system designs. Consequently, little technical understanding has been achieved in determining the relative importance of system variables and functional margins. Individual measurements of heat, light, pressure, and burning particles cannot be adequately extrapolated to a system-level performance. For example, the large volumes in all of the above-mentioned test apparatuses are not representative of actual applications. The volumes reduce the available pressure and allow heat to dissipate, even to the point of quenching the burning particles.
Generally, the known methods and apparatuses discussed above only provide relative comparisons, which often depend on operator judgment. These tests are incapable of measuring ignition within the first millisecond.
Ignitability has particular significance to aerospace applications where there is a need to measure how well an initiating element can ignite other materials. Specifications require that igniters fire when supplied with sufficient electrical or mechanical input energy. For some devices, such as the NASA standard initiator, (NSI), output is controlled by specifying pressure performance limits when fired into a closed volume (known as a "bomb") with no requirement for ignition by its output. However, no military specification or other generally accepted tests exist for measuring primer output, which is used only for ignition.
One specific test method and apparatus was developed for measuring energy output for many different kinds of explosive devices, including percussion primers. The apparatus included a piston/cylinder energy sensor containing a strength-calibrated, crushable honeycomb element. The output of a percussion primer was measured as the work done in stroking the piston to crush the honeycomb. Output was equated to the distance crushed multiplied by the crush strength of the honeycomb, and was expressed in units of inch-pounds.
Significantly, the most important percussion primer output parameter is not energy output to stroke a piston, but is instead the ignitability performance. A need exists to develop a test method and apparatus which will determine whether the primer will ignite the next item in the train, and with what margin.